[Bug ada/123945] New: Mutable tagged types specified as generic formal types can not be used for generic formal objects

[liam at liampwll dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123945

            Bug ID: 123945
           Summary: Mutable tagged types specified as generic formal types
                    can not be used for generic formal objects
           Product: gcc
           Version: 15.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
          Assignee: unassigned at gcc dot gnu.org
          Reporter: liam at liampwll dot com
                CC: dkm at gcc dot gnu.org
  Target Milestone: ---

See the below example which generates the error "expected type derived from
"T3b" defined at unknown location". Possibly related to PR/123306.

pragma Extensions_Allowed (All_Extensions);

procedure Example is
   type Mutably_Tagged is tagged null record with Size'Class => 128;

   type Normal_Tagged is tagged null record;

   generic
      type T is private;
      X : Mutably_Tagged'Class; --  Works
      Y : T; --  Error
   package G is
   end G;

   package My_G is new
     G
       (T => Mutably_Tagged'Class,
        X => Mutably_Tagged'Class (Mutably_Tagged'(null record)),
        Y => Mutably_Tagged'Class (Mutably_Tagged'(null record)));
begin
   null;
end Example;